1. Field of the Invention
This invention relates to electrostatic transducers, and in particular to electrostatic emitters operable within the ultrasonic frequency range.
2. Prior Art
Electrostatic emitters have long been utilized to generate compression waves, both in the sonic and ultrasonic frequency ranges. An advantage of this type of speaker is the low mass diaphragm which is less susceptible to distortion caused by the substantial mass of conventional acoustic speakers which rely on the large mass of a magnetic core for generation of compression waves. In view of this feature, many attempts have been made to develop an effective acoustic speaker which supplies full range audio output, yet operates within the very slight diaphragm displacement characteristic of an electrostatic emitter.
The development of an electrostatic acoustic speaker which provides strong bass or low frequency range has been most challenging. Because of the limited range of electrostatic forces, diaphragm displacement must be correspondingly limited. To allow the diaphragm to extend beyond the effective force field of the electrostatic source would result in severe distortion as the tension required within the diaphragm collapses. Therefore, diaphragm displacement is generally limited to distances measured in micrometers. Although such minimal movement may be suitable for high treble ranges, bass response requires electrostatic radiating elements to embody extremely large surface areas to generate sufficient air movement to satisfy listener taste for a strong, low bass output. Such large surface areas, however, are not only difficult to design, but require large wall space within the listener's field of use. This is particularly problematic in view of current market trends for smaller sized acoustic systems which can be concealed, yet generate large sound output.
Among the most common applications of electrostatic emitters is the propagation of ultrasonic radiation, generally for industrial applications such as material testing, range finding, and other utilities not associated with audible sound systems. For example, range detecting devices associated with automatic cameras typically rely on a small electrostatic transmitter comprising a rigid, conductive plate of approximately one inch in diameter. The plate has a series of concentric channels cut into its face to provide displacement space for a thin film of metalized Mylar superimposed over the top of the resulting ridges. When the nonconductive Mylar film is biased in tension against the plate, ultrasonic emissions can be transmitted by applying an ultrasonic frequency to the conductive metalized side of the film, thereby vibrating the film sections which are movable between the ridges. A timing circuit calculates an object distance from the camera based on time delay of the reflected signal and adjusts lens focus accordingly. Such applications are generally represented by U.S. Pat. No. 4,439,642 of Reynard.
The intended application of the ultrasonic emitter will usually determine the ultrasonic frequency range to be applied. This range starts above the audio peak at approximately 20 kHz and extends well into the MHz range. FIG. 1 illustrates the absorption effect of ultrasonic radiation in air, as a function of frequency. It will be noted that severe attenuation occurs at frequencies in the hundred kHz range and up. Therefore, very little utility has been perceived for high frequency ultrasonic applications where sound must be transmitted over significant distances. Applications for the high range frequencies is generally limited to materials testing situations where the emitter and detector are positioned proximate to the material surface. This technology generally illustrated by U.S. Pat. Nos. 4,695,986; 4,887,246; and 4,888,086 by Hassack et al, and by U.S. Pat. No. 4,429,193 of Busch-Vishniac et al.
U.S. Pat. No. 5,287,331 by Schindel illustrates design considerations for extremely high ultrasonic frequency requirements using an electrostatic film. This emitter incorporates a modified electrostatic design developed for operation in the range of ultrasonic emissions up to several MHz. The patent teaches the use of small pits formed in selected shapes and sizes (approximately 40 micrometers). Performance of the electrostatic emitter relies on closed pits which trap air between the rigid substrate and the film diaphragm. The film is retained in this sealed configuration at the pitted surface by suction arising from depressing the film into the cavity structure, and allowing the resulting suction of the closed pit cavities to retain the film in sealed contact.
Although none of the prior art electrostatic emitters suggest beneficial use as a small speaker operable within the low audio range, development of audio from heterodyne interference of two ultrasonic beams is able to generate such low frequency output. Prior art applications of this heterodyning or parametric speaker technology has focused primarily on use of bimorf piezoelectric transducers as the emitter source. These devices offer broad range operation; however, they often lack the sensitivity of an electrostatic system. Despite the many years of research endeavoring to develop a parametric speaker array capable of commercial competition with conventional acoustical speakers, no application of electrostatic emitter design using a thin film has been successfully achieved.
A brief outline of prior art development of parametric speaker design will be helpful to an understanding of the present invention which realizes effective utilization of an electrostatic system for audio sound generation. A general discussion of this technology is found in "Parametric Loudspeaker--Characteristics of Acoustic Field and Suitable Modulation of Carrier Ultrasound", Aoki, Kamadura and Kumamoto, Electronics and Communications in Japan, Part 3 Vol. 74, No. 9 (March 1991). Although technical components and the theory of sound generation from a difference signal between two interfering ultrasonic frequencies is described, the practical realization of a commercial sound system was apparently unsuccessful. Note that this weakness in the prior art remains despite the assembly of a parametric speaker array consisting of as many as 1410 piezoelectric transducers yielding a speaker diameter of 42 cm. Virtually all prior research in the field of parametric sound has been based on the use of conventional ultrasonic transducers, typically of bimorph character. The rigid piezoelectric emitter face of such transducers has very little displacement, and is accordingly limited in amplitude. Furthermore, there appears to be no unique design consideration given to the housing casement or position configuration for the emitters.
U.S. Pat. No. 5,357,578 issued to Taniishi in October of 1994 introduced alternative solutions to the dilemma of developing a workable parametric speaker system. Hereagain, the proposed device comprises a transducer which radiates the dual ultrasonic frequencies to generate the desired audio difference signal. However, in this patent the dual-frequency, ultrasonic signal is propagated from a gel medium on the face of the transducer. This medium 20 "serves as a virtual acoustic source that produces the difference tone 23 whose frequency corresponds to the difference between frequencies f1 and f2." Col 4, lines 54-60. In other words, this 1994 reference abandons direct generation of the difference audio signal in air from the face of the transducer, and depends upon the nonlinearity of a gel medium to produce sound. This abrupt shift from transducer/air interface to proposed use of a gel medium reinforces the perception of apparent inoperativeness of prior art disclosures, at least for practical speaker applications attempted to date.
What is needed is a system that provides a strong audio output, including frequencies within the bass range, which can be developed by acoustic heterodyning from an electrostatic speaker operating at frequencies within the ultrasonic range.